Other areas to hit for maximal impact on sound. Any large expanse of sheet metal such as the roof, back wall of the cab, and insides of the outer sheet metal of the doors. Those areas can be easily hit without much tear down. For a complete job you will need to do allot more tearing down to get at all the insides of the exterior sheet metal. I'd recommend asking questions on a car audio forum to gauge how much work it may take to get the level of road noise deadening you desire.

For when I change my truck into a micro RV I'll be sound insulating it. I haven't decided on how extensive I'll attack it, but I may even go in and do the firewall behind the dash. At the minimum I'll do the floor, doors, roof, and cab back wall. Doing the sound proofing also gives me the excuse of going in and cleaning up any interior rust spots.